1. Field of the Invention
This invention relates generally to video signal processing techniques for multimedia systems and, more particularly, to a method and apparatus for digitally merging video data signals from multiple sources in a single display.
2. Description of Related Art
Various techniques for displaying at least two different displays of information on a video display have been disclosed in the art. Such techniques have been most commonly disclosed in connection with the development of techniques for overlaying information on a video display. For example, U.S. Pat. No. 4,200,869 to Murayama et al. discloses a system where an alphanumeric character may be superimposed over a graphical character in a manner such that the alphanumeric character may be read but the graphical character, and thus the continuity of the graphical display, is not destroyed. In Murayama et al., the output from first and second display generators are provided to an OR gate, the output of which drives a CRT. Selective prioritizing of the generated display signals is not possible in the Murayama et al. system.
Another video display system having a signal overlay capability is disclosed in U.S. Pat. No. 4,599,610 to Lacy. In Lacy, an overlay video signal, most commonly, an alphanumeric display, is given priority over a normal video signal, most commonly, a graphical display. The normal video signal is supplied to the base of a first emitter follower transistor while the overlay video signal is supplied to the base of a second emitter follower transistor. The two transistors are biased such that the first (normal) transistor is at a slightly lower voltage than the second (overlay) transistor and is reversed biased (off) any time data is available from the overlay video signal, thereby prioritizing the display of the overlay video signal, when available, over the normal video signal.
On the other hand, to successfully merge two video images by substituting the second video image in place of the first video image poses different problems, particularly when the merging of the encoded video signals which correspond to the video images is contemplated. In order to switch from the first video signal to the second video signal while forming a merged video signal, the merged video signal will propagate the last color of the first video signal until the second video signal is available in response to a switching signal. As a result, a distinctly visible border will be formed at the transition between the first and second video images.
To avoid this problem, analog techniques have been traditionally utilized to merge video images. First and second video signals are decoded into their analog red, green and blue (or "RGB") components by respective digital-to-analog (or "D/A" converters included as part of respective digital video decoders. The first and second RGB signals are then merged at the image level using an analog switching scheme. Several disadvantages are associated with such methods. First, noise may be introduced into the displayed image since merging images by switching at the image level requires very low level analog signals. Second, in order to achieve switching at the image level, parallel video signal processing systems, including digital video signal decoders and D/A converters, are required, thereby increasing both the size and expense of such systems.